As a polishing process for improving the surface roughness of a processing surface of a workpiece and imparting a mirror finish, gloss, and so on to the processed surface, usually, for example, polishing with polishing paper or polishing cloth, polishing with buff, lapping, polishing by contact with rotating abrasive grains, or polishing by contact with abrasive grains subjected to ultrasonic vibration is performed, but blasting is not employed.
The reason that the blasting is thus not used for the polishing process such as mirror finishing and gloss finishing is that, since the blasting is a process in which an abrasive is ejected or projected onto a workpiece to cause the abrasive to collide with the processing surface of the workpiece, pearskin-like irregularities are formed on the surface of the workpiece when the abrasive collides with the workpiece surface.
In order to prevent the formation of such pearskin-like irregularities and, for example, to cut the surface of the workpiece with high precision, it has been suggested to perform blasting using fine abrasive grains having a grit size of about No. 3000 (4 μm).
However, when such fine abrasive grains are directly projected, since the mass of individual abrasive grains is small, the abrasive grains are suspended in the air in an ejection chamber formed in a cabinet of a blasting machine, thereby obstructing the view. Therefore, the processing site cannot be observed, and thus processing cannot be accurately performed.
In addition, in the case where the fine abrasive grains are used, a large amount of abrasive grains adhere to the inner surface of the cabinet and the workpiece if the abrasive grains are charged with static electricity. In order to remove this, ion air blowing and wet cleaning are necessary, and, therefore, the blasting machine is required to be provided with a device configuration for performing these, and also blasting is interrupted during the removal operation, resulting in a decrease in productivity.
Accordingly, there is a demand to develop an abrasive that is not suspended in an ejection chamber and does not cause adhesion due to static electricity to the inner surface of a cabinet and a workpiece and yet that can perform processing equivalent to that when the above-described fine abrasive grains are used.
Thus, ordinarily, a processing surface of a workpiece cannot be processed to a glossy surface such as a mirror finish by blasting, but blasting methods that allow polishing of a processing surface of a workpiece by preventing the formation of a pearskin-like texture on the processing surface of the workpiece have been proposed.
For example, in Japanese Patent No. 2957492, a grinding method in which an abrasive composed of abrasive grains attached to a carrier made of porous plant fiber having elasticity using the fat or sugar contained in the plant fiber as an adhesive is mixed with a grinding liquid; the mixture is ejected obliquely towards a surface of a workpiece so that a large number of grains collide with the surface; and the abrasive is made to slide on the processing surface of the workpiece while plastically deforming the carrier to finish the processing surface of the workpiece with the grinding material has been proposed.
Furthermore, in Japanese Patent KOKAI (LOPI) No. 2001-207160, a polishing method in which an abrasive composed of a core having a desired elasticity and adhesiveness as a result of containing water, such as gelatin, and a plurality of abrasive grains stuck to the surface of the core due to the adhesiveness is used, and the abrasive with the core thereof holding water is ejected towards a workpiece so as to collide with and polish the processing surface of the workpiece has been proposed.
In addition, in Japanese Utility Model KOKAI (LOPI) No. S 55-98565, as an abrasive that enables grinding as described above, a granular abrasive having one or a plurality of abrasive grains and an elastic material that is integrally bonded to the abrasive grains and has a restitution coefficient higher than that of the abrasive grains, such as rubber or an acrylic resin, has been proposed.
Furthermore, in Japanese Patent No. 3475252, though it has not been applied to the abrasive field yet, an adhesive elastic material of a crosslinked polyrotaxane compound has been proposed.
Here, the “polyrotaxane” has a structure in which a straight-chain molecule cannot be drawn out from circular molecules due to blocking groups bonded on both ends of the straight-chain molecule passing through open portions of a plurality of the circular molecules (see FIG. 2), and the “crosslinked polyrotaxane” is one crosslinked by chemical bonding between the circular molecules of a plurality of the polyrotaxane molecules (see FIG. 3). In this crosslinked polyrotaxane, no direct crosslinking point is present between the straight-chain molecules, and thereby the straight-chain molecule can move in the open portions of the circular molecules. Therefore, when a stress is applied to such a compound, the crosslinking point moves by the movement of the straight-chain molecule in the open portions of the circular molecules [see FIGS. 3(A) and (B)]. As a result, the internal stress of the compound is dispersed to thereby give higher breaking strength, elasticity, and resilience, and also the straight-chain molecules form a network structure to thereby give a high swelling property.